The
Garden Magazine 32(2): 98-99 (Oct 1920) Do Plants Need the Dark?Strangely Dependent Upon It They Assuredly Are, According To the Latest
Discoveries
That Follow Exhaustive Investigation and Experiment on the Part of
Government Experts

NOTHING in the world is more dramatic than the denouement which often comes at the
close of a long series of patiently conducted experiments, that have been
carried on sometimes so gropingly that the layman finds himself wondering if
the experimenter has any idea at all of what he is trying to do! And dramatic
is the result at which careful workers have arrived in recently completed
experiments, covering a period of two years, concerned with the effect on
plants of light. Everyone—scientist and layman—has believed for generations
that sunlight was necessary for the normal growth of most kinds of plants; but
no one has ever supposed that the darkness played any especial part in the
process.

Yet
it is now demonstrated that, quite apart from possible injury from burning by a
too hot sun, plants may be adversely affected by too much daylight; or, in
other words, too many hours of daylight in proportion to the number of hours of
darkness which they enjoy. Too long a day as well as too short a day, will
prevent many kinds of plants from ever arriving at the stage of flowering and
fruiting. It has also been found that, although daylight in too great
proportion for flowering and fruiting may stimulate profuse vegetative growth,
plants will not reproduce except when exposed to a favorable length of day.
Length of day favorable to both reproduction and growth seems to be the factor
that results in the "ever-bearing" type of fruits.

Plant
life depends, in short, on light rather than temperature for growth, and may be
controlled by regulating the hours of light and darkness. The principle is
revolutionary; but it rests on actual experiments in which it was demonstrated
that plants subjected to alternate periods of light and darkness in carefully,
determined proportions, could be brought to maturity at any time of the year.
Going even further, the experts say that eventually it may be found that the
animal organism also is capable of responding to the stimulus of certain day
lengths. They believe that the migration of birds may be an illustration, on
the ground that direct response to such a stimulus would be more in line with
modern biological teachings than theories which assume that birds "go
south" as a matter of instinct.

MAKING
ASTERS BLOOM TO ORDER

A
DECORATIVE RADISH

Note
the superior vegetation of plants which lived a normal out-of-doors
existence, but show no desire to bloom (on right), whereas those under
artificial light treatment (on left) are already in full blossom

Shorter
days meant a longer—
and more decorative—seedstalk!

Furthermore,
it has been shown conclusively that the intensity of the light has very much
less influence upon the growth of the plant than has usually been supposed.
Rather it is the number of hours; for the experiments show that the flowering
and fruiting period of practically any plant can be made to take place at any
time of the year by darkening the greenhouse in the morning and evening if the
day is too long, or by lengthening the day by artificial light if the day is
too short. This application of the newly discovered principle will, of course,
be used by florists and other greenhouse operators. For example, Violets bloom
naturally only during the comparatively short days of spring; but if Violet
plants are covered with light-proof boxes at night and not uncovered until the
sun is about "half a house" high each morning during the summer time,
they can be forced to bloom again in the summer.

Spring
flowers and spring crops happen to be spring flowers and spring crops because
the days at the season of their flowering have the necessary proportion of
hours of daylight to bring these particular things to flower or to maturity at
this time. Correspondingly, the early summer flowers and crops must have a
longer period of daylight. This has been proven as to a large number of plants;
and scientists believe that the principle will hold not only throughout the
higher forms of plant life, but that it is probably applicable to animal life
as well.

Long
series of tests have been made with a large variety of vegetables and flowers.
By employing dark chambers to shorten the period of light and artificial lights
to extend it, they have shortened or lengthened the life cycle of plants and
forced some of them to complete two cycles in one season. They have brought
others into flower and fruit months in advance of their regular time and, with
still others, have greatly delayed and even completely prevented fruiting.

A
test made with Soy Beans is typical of the experiments carried on and shows how
the principle works. Both test and control plants were used in order to check
up results. For the test plants the day was shortened by several hours. That
is, they were exposed to the light only from 10 o'clock in the morning until 3
o'clock in the afternoon. They were placed in the dark house May 20, 1919. The
control plants, otherwise treated exactly like the test plants, were left
exposed to the light from dawn until dark. The first blossoms appeared on the
dark-house plants on June 16. On the plants that were left in the light all day
no blossoms appeared until September 4, or 80 days later. But, while the
dark-house plants averaged only 6 or 7 inches in height, those that were left
in the light all day grew to an average height of 57 or 58 inches. This test
proved conclusively that the Soy Bean requires a short day and a long night for
flowering and seed bearing.

In
tests with other plants, just the opposite was found to be true. The plants
that were left in the light all day did not grow luxuriantly, but produced
flowers and seeds, while those that. were kept in the dark a part of the day
made abundant growth. The latter, however, were either greatly retarded in
producing seed or produced none at all. Temperature moreover, appeared to exert
no influence in the tests. A striking illustration of the relative unimportance
of temperature lies in the fact that plants kept in the dark for a part of the
day underwent, in midsummer, the changes that naturally come in the fall and
that, heretofore, have been attributed to lower temperatures. This was true
even when the dark houses registered a higher temperature than the outside
temperature.

Equally
interesting were the results obtained by artificially extending the period of
light instead of shortening it. In a test with Ins, for example, the artificial
illumination was so arranged as to give 18 hours of continuous light in a
greenhouse during the winter, a large number of tungsten filament incandescent
lights being used for the purpose. Control plants were kept in a similar
greenhouse with no artificial light. This test was begun on October 20, 1919.
In the greenhouse where daylight was supplemented with electric light the
plants made rapid growth, soon attained normal size and produced blossoms on
December 24. On the other hand, the plants in the greenhouse where no
artificial light was used, remained practically dormant and showed no tendency
to blossom as late as February 12, 1920, notwithstanding that this greenhouse
was kept at the same temperature as the other.

Another
test was conducted with the seed of Spinach, which was sowed November 1, 1919,
and came up in both greenhouses on November 6. The plants in the control house,
20 to 25 in number, grew very slowly, producing low, compact, leafy growths or
rosettes, and gave no evidence of blossoming as late as February 12, 1920. The
plants in the lighted house, however, elongated very rapidly, soon developed
flower stalks, and all blossomed in the period between the dates of December
8 and 23, 1919. These continued to elongate more or less throughout January and
February, 1920, blossoming and shedding pollen continuously, thus becoming in
effect "ever-blooming" plants.

The
advance in agricultural practice which may come through this new discovery will
have to be brought about largely by plant breeders and other crop specialists.
For instance, it will prove of material significance in the future planning of
cropping systems for different regions, especially where consideration of new
crops from different latitudes is necessary. The new principle undoubtedly
explains the erratic behavior which has been observed with many crops when they
are shifted to different latitudes, and may also clear up the conflicting
results of variety tests and field tests conducted with the same crops in
different regions.

The
experiments have shown, for instance, that Ragweed requires for flowering a
stimulus that is afforded by the shortening of the days and lengthening of the
nights. It does not come into flower until the period of daylight falls below
15 hours. In the latitude of Washington D. C., that comes about July 1. But if
Ragweed should be taken to northern Maine and planted, the plants would not
experience a proper length of day until after August 1 and, though the
vegetative growth might be very rank, they could not mature seed before killing
frosts intervened. The long days, therefore, might make it impossible for
Ragweed to perpetuate itself in that latitude. On the other hand, plants that
get their flowering stimulus from a long day could not perpetuate themselves
through seed formation at the equator, where the day never exceeds 12 hours.

This
principle seems to clear up the puzzling fact that many plants grow most
luxuriantly near the northern limit of their range. The long northern day
allows them to attain their maximum growth before the shorter day intervenes to
check vegetative growth and start the reproductive process.

RADISHES
ARE DECIDED LOVERS OF LIGHT

AND
THE SOY BEAN IS JUST THE REVERSE

This
is conclusively proved by the response of plants (on left) raised under usual
conditions, and the marked dejection of those (on right) whose light supply
was restricted to seven hours daily

The
almost tropical abundance of the plants (on right) matured in comparative
night (their daily light-exposure was only from ten to three) convincingly
indicates the magical properties of darkness